Gas turbine engines include a turbine disposed aft of a combustor for extracting energy from the hot, high velocity combustion gases passing from the combustor. The turbine includes a plurality of rotor stages attached to a rotatable shaft for driving a compressor for delivering pressurized air to the combustor. Each rotor stage includes a plurality of turbine blades that extend outwardly from a rotor disk. The outboard tips of the turbine blades terminate in close proximity to a facing seal carried on a stationary stator shroud circumscribing the turbine rotor assembly.
Maintaining a minimal clearance between the tips of the turbine blades and the facing surface of the seal over the entire operating range of the gas turbine engine is desirable to minimize leakage of combustion gases past the turbine blades. Excessive leakage results in a decrease in turbine performance, causing specific fuel consumption to increase and power output to decrease. However, the rotating blades of the turbine are subject to differential expansion and contraction relative to the stationary stator shroud, on which the tip seals are carried, over the operating range of the engine as the temperature and mass flow of the combustion gases passing through the turbine fluctuates amongst steady state operation at cruise, and transient conditions during rapid acceleration at low power operation, such as idle and taxi, or at high power operation, such as take-off, climb and high-speed maneuvers.
It is customary practice during operation of high bypass turbofan gas turbine engines on large commercial aircraft to control turbine blade tip seal clearance by selectively cooling the outboard surface of the turbine case shell during transient conditions in which expansion of the turbine rotor assembly is expected. In high bypass turbofan gas turbine engines, a fan air duct is defined between a relatively large diameter outer fan cowl and an inner fan cowl disposed radially inboard of the outer fan cowl. The inner fan cowl circumscribes the engine core casing and establishes a cavity about the engine core casing that is isolated from the relatively cold air stream passing through the fan air duct. Thus, the engine core casing, which includes a section circumscribing the turbine, is bounded by a relatively warm, low pressure, dead air cavity. Control of turbine blade tip seal clearance is typically achieved in such high bypass turbofan gas turbine aircraft engines by selectively cooling the exterior surface of the engine core casing circumscribing the turbine when it is desired to cause the section of the engine core casing circumscribing the turbine to shrink, such as during steady-state operation, thereby causing the seals facing the tips of the rotating turbine blades to move inboard to lie in closer proximity to the turbine blade tips. Typically, the cooling air is drawn from the fan air stream.
However, smaller commercial and non-commercial aircraft are commonly equipped with non-cowled gas turbine engines, such as moderate and low bypass turbofan gas turbine engines. Typically, on moderate and low bypass gas turbine engines, there is no inner fan cowl isolating the engine core casing from the fan air stream. Rather, the section of the engine core casing circumscribing the turbine is directly exposed to the relatively cold fan air stream passing through the fan duct. Consequently, it is generally not necessary to selectively cool the turbine case shell during steady-state operation of a non-cowled gas turbine engine. However, during transient operations such as take-off, climb or other rapid engine acceleration event, there exists a risk of the turbine blades expanding and contacting the tip seals carried on the turbine shroud, which is supported from the section of the engine core casing circumscribing the turbine, because the turbine shroud remains relatively cooler than the turbine blades due to the engine core casing being in direct contact with the relatively cold fan air stream.
Therefore, it would be desirable to provide a method and system for controlling seal clearance between the turbine blades and the blade tip seals during periods of transient operation of a gas turbine engine on a non-cowled gas turbine engine.